1. Field of the Invention
The present invention relates to an automatic mold thickness adjusting apparatus of an injection molding machine that can automatically adjust a mold thickness in the injection molding machine.
2. Description of the Related Art
To obtain a good product by an injection molding machine, it is important to maintain a clamping force at a predetermined value. A mold attached to the injection molding machine is operated in a state in which the mold is heated and the temperature thereof is raised, and the rise in temperature makes the mold thermally expand. Therefore, consideration has to be given to the thermal expansion of the mold when adjusting the clamping force.
Japanese Patent Application Laid-Open No. 4-103311 discloses a technique of detecting and storing a mold temperature at the time of mold thickness adjustment that is performed at a low ambient temperature when adjusting a clamping force, detecting the temperature of the mold at every molding cycle after the start of the molding cycle, comparing the temperature with the mold temperature at the time of previous mold thickness adjustment, and, when the difference is greater than or equal to a set value, continuing the molding cycle after performing a mold thickness adjustment again.
The problem of the technique disclosed in Japanese Patent Application Laid-Open No. 4-103311 described above is that, at a molding cycle, when the difference between the detected mold temperature and the mold temperature at the time of mold thickness adjustment is greater than or equal to a set value, despite the mold thickness adjustment performed at a low ambient temperature, the mold thickness adjustment is performed again, which requires much time to perform an adjustment, resulting in a prolonged equipment shutdown time and low productivity.
Moreover, another problem is that, since the mold thickness adjustment is performed after the molding cycle is performed, the position of a rear platen is adjusted after clamping is performed once in a state in which the mold has thermally expanded due to a rise in temperature, and therefore, in a state before the position of the rear platen is adjusted, an excessive clamping force is applied to the mold and an excessive load is put on the injection molding machine, which significantly reduces the longevity of the machine.